1. Field of the Invention
This invention relates to a sheet structure which includes a layer of polyolefin synthetic pulp. More particularly, the invention concerns a calendered composite sheet which includes a scrim of continuous filaments adhered to the layer comprising the polyolefin synthetic pulp.
2. Description of the Prior Art
Synthetic pulps made from polyolefins, such as polyethylene, polypropylene and blends thereof, are known in the art. Kirk-Othmer: Encyclopedia of Chemical Technology, volume 19, third edition, John Wiley & Sons, p. 420-435 (1982) describes synthetic pulps as generally being very fine, highly branched, discontinuous, water-dispersible fibers made from plastics. Known methods for producing the synthetic pulps include solution flash-spinning, emulsion flash-spinning, melt extrusion/fibrillation and shear precipitation. The pulps may be blended with woodpulp or glass fibers or other short fibers and made into papers, sheets or boards by conventional wet-lay papermaking techniques. The pulps may include various additives such as wetting agents (e.g., polyvinyl alcohol), inorganic fillers (e.g., kaolin, clay, talc, calcium carbonate), stabilizers and the like. The polyolefin pulp products so produced may be heated to fuse the polyolefin pulp, and, as disclosed by Hercules Incorporated, Bulletin LT-109 "Pulpex.RTM. Polyolefin Pulps for Nonwovens" (March 1982), are suited for use as bonding agents for certain nonwoven materials such as dry-laid, Rando-Webber-formed sheets and wet-laid, Fourdrinier-formed sheets. In such nonwoven materials, the polyolefin pulps are blended with pulps of wood fibers or waste fibers, the polyolefin being in amounts that generally range from 2 to 40% of the total blend. The porosity and bondability of such materials can be controlled by adjusting the temperature and pressure of bonding as well as the fraction of polyolefin pulp included.
The present inventor has found that the utility of the above-described types of synthetic-pulp nonwoven materials is limited. The sheets often lack strength and need considerable improvement in durability and abrasion resistance of the sheet surface. Accordingly, an object of the present invention is to provide a sheet structure wherein the shortcomings associated with polyolefin synthetic pulp sheets are minimized or at least somewhat alleviated.
Many types of composite nonwoven sheets are known in the art. For example, although not related to synthetic pulps, Research Disclosure, 15789 "Reemay.RTM. paper composites" (June 1977) discloses that a spunbonded continuous filament polyester nonwoven fabric can be combined with wet-laid wood pulps, staple fibers and resins to form composite sheets. With or without resins, the composite sheets are disclosed to have improved properties relative to the paper, e.g., 4 to 7 times the tear strength, 3 to 4 times the dimensional stability on exposure to moisture, 2 to 4 times the wet strength and improved fold resistance. Also disclosed is the use of adhesive resin within the pulp or as a coating on the continuous filament nonwoven fabric to improve the adhesion of the pulp layer to the nonwoven fabric.